An organic electroluminescence device (hereinafter the term “electroluminescence” is often abbreviated as “EL”) is a self-emission device utilizing the principle that a fluorescent compound emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is impressed.
As the stacking structure of the organic EL device, a two-layered type of a hole-transporting (injecting) layer and an electron-transporting emitting layer, and a three-layered type of a hole-transporting (injecting) layer, an emitting layer and an electron-transporting (injecting) layer are widely known. In such stack structure devices, their device structures and fabrication methods have been contrived to increase recombination efficiency of injected holes and electrons.
Conventionally, as the hole-transporting material used in an organic EL device, an aromatic diamine derivative disclosed in Patent Document 1 or an aromatic condensed ring diamine derivative disclosed in Patent Document 2 are known.
However, in an organic EL device in which such aromatic diamine derivative is used in a hole-transporting material, in order to obtain a sufficient luminance, a higher voltage is required to be applied. As a result, a problem arises that device life is shortened or power consumption is increased.
As a method for solving these problems, a method in which an electron-accepting compound such as a Lewis acid or the like is doped with a hole-injecting layer of an organic EL device is proposed (Patent Documents 3 to 9 or the like). The electron-accepting compounds used in Patent Documents 3 to 6 have, however, problems that they are unstable when handled during the production process of an organic EL device or stability such as heat resistance becomes insufficient at the time of driving an organic EL device, resulting in a shortened device life.
Tetrafluorotetracyanoquinodimethane exemplified in Patent Documents 3, 4 or the like as an electron-accepting compound has a small molecular weight and has a high degree of sublimation properties since it is substituted by fluorine. Therefore, it may be diffused within an apparatus when fabricating an organic EL device by vacuum vapor deposition, thereby to contaminate the apparatus or the device.
The inventors made intensive studies on electron-accepting compounds or the like. As a result, the inventors noticed a fluorenone derivative. A fluorenone derivative is known as an electron-transporting material of an electrophotographic photoreceptor (see Patent Documents 10 and 11).
A thioether group or the like is introduced into this derivative in order to improve compatibility with a binder resin or solubility in a solvent, which is a subject to be solved as an electrophotographic photoreceptor. Therefore, there is a possibility that decomposition or the like of the thioether portion may occur by a deposition process which is required for fabricating an organic EL device or by the Jule heat or the like which is generated at the time of driving a device. That is, this derivative is concerned for the lowering of electron acceptability or other problems when applied to an organic EL device, even though it is satisfactory as an electrophotographic photoreceptor.    Patent Document 1: U.S. Pat. No. 4,720,432    Patent Document 2: U.S. Pat. No. 5,061,569    Patent Document 3: JP-A-2003-031365    Patent Document 4: JP-A-2001-297883    Patent Document 5: JP-A-2000-196140    Patent Document 6: JP-A-H11-251067    Patent Document 7: JP-A-H04-297076    Patent Document 8: JP-T-2004-514257    Patent Document 9: US2005/0255334A1    Patent Document 10: JP-A-2005-121887    Patent Document 11: Japanese Patent No. 3670481
The invention has been made based on the above-mentioned problems, and an object thereof is to provide an electron-accepting material suitable as the material constituting an organic EL device, in particular as a hole-injecting material.